1. Field of the Invention
The present invention relates to back light units for a liquid crystal display devices. More particularly, the present invention relates to a back light unit having a holographic pattern layer.
2. Discussion of the Related Art
Active research has been conducted relating to flat display devices. The research has been most focused on Liquid Crystal Display Devices (LCDs), Field Emission Display Devices (FEDs), Electro-luminescence Display Devices (ELDs), and Plasma Display Panels (PDP). Of the flat display devices, a field of application for liquid crystal display devices expands to notebook PCs, desk top monitors, and liquid crystal TVs including features such as high contrast ratio, suitability for expression of gray scales and motion pictures, and low power consumption.
However, because the liquid crystal display device is not luminous, the liquid crystal display device requires an external light source for emitting a light. Particularly, in a case of a transmissive liquid crystal display device, a separate illuminating device for emitting and guiding the light to a back surface of the LCD panel, i.e., a back light unit, is required, invariably.
Back light units include an edge type and direct type based upon the methods of projecting the light. The edge type back light unit has a tube-type linear light source, such as a lamp light, at a side of the liquid crystal panel, to project the light from the lamp light to the liquid crystal panel throughout an entire surface thereof. The direct type back light unit has lamp lights selectively mounted under the liquid crystal panel to distribute the light from the lamp lights to the liquid crystal panel throughout an entire surface thereof, by diffusing the light by means of a diffusion sheet located between the lamp lights and the liquid crystal panel.
Since no light plate is required, the direct type back light unit is advantageous in that the direct type back light unit is suitable for fabrication of a thin, and light weight liquid crystal display device that provides a high luminance and uniform light distribution.
A related art direct type back light will be described with reference to the attached drawings.
FIG. 1 illustrates an exploded perspective view of a related art direct type back light unit, and FIG. 2 illustrates a section across I-I′ of FIG. 1, and FIGS. 3A and 3B illustrate SEM photographs of diffusion sheets.
A liquid crystal display device having the back light unit as a basic element will be described.
Generally, the liquid crystal display device includes a liquid crystal panel having opposite upper and lower substrates bonded together with a liquid crystal layer inbetween, polarizing plates respectively attached to upper, and lower surfaces of the liquid crystal panel for transmitting light of only one direction, a direct type back light mounted under the liquid crystal panel for providing a light to the liquid crystal panel, a case for covering an outside surface of the back light unit to support the liquid crystal panel and the direct type back light unit, and a bezel part of stainless steel attached to an outside of the case for covering a circumference of the liquid crystal panel except an effective area where a picture is to be displayed thereon.
In FIGS. 1 and 2, the direct type back light is provided with a lamp light 111 for providing a light, and on optical sheet having a diffusion sheet 115, a prism sheet 116, and a protective sheet 117 between the light lamp 111 and the liquid crystal panel 110.
In general, there is a reflective plate (not shown) under the light lamp 111 for directing the light from the light lamp 111 toward the display portion of the liquid crystal panel.
The lamp light 111 may be a flat lamp light or a Cold Cathode Fluorescent Lamp (CCFL).
The flat lamp light has a discharge space formed by upper and lower boards sealed with a gap therebetween with discharge gas, such as neon Ne, argon Ar, or mercury Hg filled therein, a fluorescent material layer coated on each of opposite inside wall surfaces of the upper and lower boards, and an electrode formed on the lower board for applying power thereto.
Since a certain space, and adequate optical sheets are required between the lamp light and the liquid crystal panel for preventing a shape of the lamp light from being visible on a screen, use of the CCFL imposes a limitation on fabrication of a thin liquid crystal display device. Therefore, in a case when a light source with a comparatively large display area and a uniform luminance is required, the flat lamp light will be more suitable, of which an entire surface opposite to a display face of the liquid crystal panel is luminous.
The diffusion sheet 115, the prism sheet 116, and the protective sheet 117 are collectively called the optical sheet. There are a plurality of optical sheets between the light lamp 111 and the liquid crystal panel 110 to enhance a light diffusing effect to prevent the shape of the light lamp from being visible on the display surface of the liquid crystal panel and provide a light having a uniform luminance distribution throughout the surface.
In detail, the diffusion sheet 115 uniformly diffuses the light from the lamp light, the prism sheet 116, with a plurality of triangular linear prisms, collects the diffused light, and directs the light toward the liquid crystal panel, and the protective sheet 117 on the prism sheet 116 protects the prism sheet 116. As shown in FIGS. 3 and 4, the diffusion sheet 115 has spherical light diffusing fine particles 115a on a surface for diffusing the light by differences of refractive indices.
As the light diffusing fine particles 115a, glass, polystyrene, polycarbonate, PMMA, or so on is used, which makes a light transmits, and refracts, to change a light path, and diffuse the light. The prism sheet 116 has spherical or non-spherical lenses added to a prism sheet which can collect a diffused light, for collimating the light within a predetermined angle for improving a luminance.
However, the placing of various kinds of optical sheets between the liquid crystal panel and the lamp light impairs an optical efficiency as a portion of the light from the lamp is absorbed in the optical sheets as the light passes through the optical sheets, the stacking and light combination of various kinds of optical sheets may cause a defect in view of fabrication process, and the increased thickness of the sheets is contrary to development of a thin liquid crystal display device which consumers require.